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* [RFC v2 0/6] x86: dynamic indirect branch promotion
@ 2018-12-31  7:21 Nadav Amit
  2018-12-31  7:21 ` [RFC v2 1/6] x86: introduce kernel restartable sequence Nadav Amit
                   ` (7 more replies)
  0 siblings, 8 replies; 42+ messages in thread
From: Nadav Amit @ 2018-12-31  7:21 UTC (permalink / raw)
  To: Ingo Molnar, Andy Lutomirski, Peter Zijlstra, Josh Poimboeuf,
	Edward Cree
  Cc: H . Peter Anvin, Thomas Gleixner, LKML, Nadav Amit, X86 ML,
	Paolo Abeni, Borislav Petkov, David Woodhouse, Nadav Amit

This is a revised version of optpolines (formerly named retpolines) for
dynamic indirect branch promotion in order to reduce retpoline overheads
[1].

This version address some of the concerns that were raised before.
Accordingly, the code was slightly simplified and patching is now done
using the regular int3/breakpoint mechanism.

Outline optpolines for multiple targets was added. I do not think the
way I implemented it is the correct one. In my original (private)
version, if there are more targets than the outline block can hold, the
outline block is completely removed. However, I think this is
more-or-less how Josh wanted it to be.

The code modifications are now done using a gcc-plugin. This allows to
easily ignore code from init and other code sections. I think it should
also allow us to add opt-in/opt-out support for each branch, for example
by marking function pointers using address-space attributes.

All of these changes required some optimizations to go away to keep the
code simple. I have still did not run the benchmarks again. 

So I might have not addressed all the open issues, but it is rather hard
to finish the implementation since some still open high-level decisions
affect the way in which optimizations should be done.

Specifically:

- Is it going to be the only indirect branch promotion mechanism? If so,
  it probably should also provide interface similar to Josh's
  "static-calls" with annotations.

- Should it also be used when retpolines are disabled (in the config)?
  This does complicate the implementation a bit (RFC v1 supported it).

- Is it going to be opt-in or opt-out? If it is an opt-out mechanism,
  memory and performance optimizations need to be more aggressive.

- Do we use periodic learning or not? Josh suggested to reconfigure the
  branches whenever a new target is found. However, I do not know at
  this time how to do learning efficiently, without making learning much
  more expensive.

[1] https://lore.kernel.org/patchwork/cover/1001332/

Nadav Amit (6):
  x86: introduce kernel restartable sequence
  objtool: ignore instructions
  x86: patch indirect branch promotion
  x86: interface for accessing indirect branch locations
  x86: learning and patching indirect branch targets
  x86: outline optpoline

 arch/x86/Kconfig                             |    4 +
 arch/x86/entry/entry_64.S                    |   16 +-
 arch/x86/include/asm/nospec-branch.h         |   83 ++
 arch/x86/include/asm/sections.h              |    2 +
 arch/x86/kernel/Makefile                     |    1 +
 arch/x86/kernel/asm-offsets.c                |    9 +
 arch/x86/kernel/nospec-branch.c              | 1293 ++++++++++++++++++
 arch/x86/kernel/traps.c                      |    7 +
 arch/x86/kernel/vmlinux.lds.S                |    7 +
 arch/x86/lib/retpoline.S                     |   83 ++
 include/linux/cpuhotplug.h                   |    1 +
 include/linux/module.h                       |    9 +
 kernel/module.c                              |    8 +
 scripts/Makefile.gcc-plugins                 |    3 +
 scripts/gcc-plugins/x86_call_markup_plugin.c |  329 +++++
 tools/objtool/check.c                        |   21 +-
 16 files changed, 1872 insertions(+), 4 deletions(-)
 create mode 100644 arch/x86/kernel/nospec-branch.c
 create mode 100644 scripts/gcc-plugins/x86_call_markup_plugin.c

-- 
2.17.1


^ permalink raw reply	[flat|nested] 42+ messages in thread

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Thread overview: 42+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-12-31  7:21 [RFC v2 0/6] x86: dynamic indirect branch promotion Nadav Amit
2018-12-31  7:21 ` [RFC v2 1/6] x86: introduce kernel restartable sequence Nadav Amit
2018-12-31 20:08   ` Andy Lutomirski
2018-12-31 21:12     ` Nadav Amit
2019-01-03 22:21   ` Andi Kleen
2019-01-03 22:29     ` Nadav Amit
2019-01-03 22:48       ` Andi Kleen
2019-01-03 22:52         ` Nadav Amit
2019-01-03 23:40           ` Andi Kleen
2019-01-03 23:56             ` Nadav Amit
2019-01-04  0:34   ` hpa
2018-12-31  7:21 ` [RFC v2 2/6] objtool: ignore instructions Nadav Amit
2018-12-31  7:21 ` [RFC v2 3/6] x86: patch indirect branch promotion Nadav Amit
2018-12-31  7:21 ` [RFC v2 4/6] x86: interface for accessing indirect branch locations Nadav Amit
2018-12-31  7:21 ` [RFC v2 5/6] x86: learning and patching indirect branch targets Nadav Amit
2018-12-31 20:05   ` Andy Lutomirski
2018-12-31 21:07     ` Nadav Amit
2018-12-31  7:21 ` [RFC v2 6/6] x86: outline optpoline Nadav Amit
2018-12-31 19:51 ` [RFC v2 0/6] x86: dynamic indirect branch promotion Andy Lutomirski
2018-12-31 19:53   ` Nadav Amit
2019-01-03 18:10     ` Josh Poimboeuf
2019-01-03 18:30       ` Nadav Amit
2019-01-03 20:31         ` Josh Poimboeuf
2019-01-03 22:18 ` Andi Kleen
2019-01-07 16:32   ` Peter Zijlstra
2019-01-08  7:47     ` Adrian Hunter
2019-01-08  9:25       ` Peter Zijlstra
2019-01-08 10:01         ` Adrian Hunter
2019-01-08 10:10           ` Peter Zijlstra
2019-01-08 17:27             ` Andi Kleen
2019-01-08 18:28               ` Nadav Amit
2019-01-08 19:01                 ` Peter Zijlstra
2019-01-08 20:47                   ` Nadav Amit
2019-01-08 20:53                     ` Andi Kleen
2019-01-09 10:35                     ` Peter Zijlstra
2019-08-29  8:23                       ` Tracing text poke / kernel self-modifying code (Was: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion) Adrian Hunter
2019-08-29  8:53                         ` Peter Zijlstra
2019-08-29  9:40                           ` Adrian Hunter
2019-08-29 11:46                             ` Peter Zijlstra
2019-09-12  7:00                               ` Adrian Hunter
2019-09-12 12:17                                 ` hpa
2019-01-08 18:57               ` [RFC v2 0/6] x86: dynamic indirect branch promotion Peter Zijlstra

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